Transcription of gene sequences (i.e., production of mRNA) is controlled at a number of different levels. Transcription initiation sites, or promoters, have different strengths, and the frequency of initiation of transcription of a given gene can also be augmented by enhancer sequences. Pausing during transcription can influence the rate of transcription and, hence, the amount of transcript produced in a given time period. Rates of pre-mRNA splicing, polyadenylation and cleavage can also influence the level of mRNA produced by a transcription unit. In addition, sequences within a mRNA molecule can regulate its transport from the nucleus to the cytoplasm, and its rate of turnover (i.e., its cytoplasmic stability).
Certain sequences within mRNA molecules that regulate the cytoplasmic accumulation and stability of mRNA have been identified and denoted post-transcriptional regulatory (PRE) elements. PRE sequences have been identified in the genome of human hepatitis B virus (the HPRE) and in the genome of the woodchuck hepatitis virus (WPRE). See, for example, Donello et al. (1998) J. Virology 72:5085-5092.
Expression of polypeptides (e.g., therapeutic antibodies, growth factors) in vitro is important for the pharmaceutical industry, and methods to maximize protein expression are needed.